A steering device of an automotive vehicle generally includes a rack which is mechanically coupled to a pair of right-hand and left-hand wheels through a tie rod, a steering arm, etc. When a steering wheel is rotated by the driver, the rack is moved in the axial direction so that steering of both the right-hand and left-hand wheels is performed in a united manner with the movement of the rack.
In recent years, there is proposed a method for controlling and adjusting steering of each of the right-hand and left-hand wheels independently by separating the mechanical coupling of the rack from one of the right-hand wheel and the left-hand wheel.
For example, Japanese Laid-Open Patent Application No. 2003-170849 discloses a steering device for automotive vehicle which comprises a plurality of steering actuators disposed for a plurality of wheels respectively, and a link member which associates the plurality of wheels mutually. In this steering device, steering of each of the plurality of wheels is controlled independently by using the link member.
Moreover, Japanese Laid-Open Patent Application No. 10-324253 discloses a steering angle varying device for automotive vehicle in which a rack is movable in the lateral direction of the vehicle according to a steering operation of the steering wheel, and a nut part and a thread part are provided between the rack and the wheels. The steering angle varying device is expanded or contracted by itself to change the steering angle of the wheels according to the steering operation. By adopting such steering device, it is possible to perform control and adjustment of steering according to the running conditions of the vehicle, such as turning speed.
The steering device in which steering of each of the pair of wheels is performed independently is demanded that, even if a problem arises in the actuator for steering one of the pair of wheels, the steering of all the wheels is carried out safely and reliably.
In this respect, there is no teaching in Japanese Laid-Open Patent Application No. 10-324253 as to how to perform the steering of all the pair of wheels certainly when a problem arises in the actuator for steering one wheel.
In the steering device of Japanese Laid-Open Patent Application No. 2003-170849, when a problem arises in one of the steering actuators, operation of the other steering actuator is transmitted through the link member, so that steering of both the wheels is performed. However, in the steering device of Japanese Laid-Open Patent Application No. 2003-170849, the load exerted on the link member to associate the pair of wheels mutually is heavy, and in order to withstand such heavy load, a large link member must be provided.